What is the best way to say "I love you" to a girl?

If you are not going out, then going ahead and saying "I love you", will very likely scare her off. You need to take it slow, are you sure she likes you too before you ask her out as asking her out and been rejected would do nothing for your confidence, and so you should be sure before you ask her out in the first place. Asking her out will be simple so long as you get your confidence up, you could write her a letter if you cannot pluck up the courage to ask her in person???